Ivo M. Pinatti

α-Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag2WO4. Theses α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag2-2xZnxWO4 models. The electronic properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were linked to distortion effects and oxygen vacancies (VOx) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were explained by means of distortional effects and oxygen vacancies (VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag+ cations was four.

Novel Gd(OH)3, GdOOH and Gd2O3 Nanorods: Microwave-Assisted Hydrothermal Synthesis and Optical Properties

We present a study of the controlled synthesis and optical properties of single-crystals Gd(OH)3, GdOOH and Gd2O3 nanorods. In this work, Gd(OH)3 nanorods were synthesized by a simple and fast microwave-assisted hydrothermal method. This process combined with the thermal decomposition oxidation of Gd(OH)3 nanorods as precursors enabled the preparation of single-crystalline GdOOH and Gd2O3 structures with well-defined morphology at low temperatures. The crystal structure dependence on the optical properties was investigated. We observed a green shift effect on the photoluminescence (PL) emission spectra from Gd(OH)3 to Gd2O3 nanorods, which can be attributed to different types of surface defects, as well as intrinsic properties that contribute significantly to the modified PL behavior.